Conductive films with excellent transparency are widely used for such purposes as display electrodes for flat panel displays such as plasma displays and liquid crystal displays; transparent electrodes for touch panels; transparent electrodes for solar cells; and electromagnetic wave shielding films.
Tin-doped indium oxide (Indium Tin Oxide, hereunder referred to as “ITO”) is well known as a material for forming conductive films with excellent transparency. ITO films are produced by gas phase methods such as sputtering or vacuum vapor deposition, but gas phase methods require high vacuum apparatuses and large equipment investment. The equipment investment and maintenance cost becomes particularly immense for purposes that require large-sized areas. Furthermore, since the component gas pressure in the production apparatus must be precisely controlled for production of each ITO film, this affects the production cost and productivity.
One method for solving this problem is a method of forming an ITO film by coating an ITO fine particle dispersion and drying it (PTL 1). However, ITO films are associated with numerous problems that must be overcome, such as the high cost of indium, difficulty of increasing conductivity while maintaining transparency, and susceptibility to bending.
Development of conductive films that employ conductive materials instead of ITO has been advancing in recent years. Notably, methods of forming mesh-like or other patterns with dispersions of metal fine particles or extra fine wires in solution, by screen printing, ink jet printing or utilizing a self-organization phenomenon, have been proposed (Patent documents 2-6).
In printing methods, however, productivity is reduced due to clogging of the mesh or nozzle, and it is difficult to form intricate patterns with line widths of less than 6 μm. Patterns with approximately 2 μm widths have reportedly been formed by ink jet printing, but very long time periods are necessary due to the small coating amounts used, and such methods are therefore poorly suitable for industrial-scale production. Moreover, with a self-organization phenomenon, which utilizes hydrophilic/hydrophobic interaction between the substrate and solution, this prohibits free selection of the substrate and complicates control of the pattern shape.
Photolithographic methods and methods employing electrodeposited meshes have also been proposed.
In one of the proposed photolithographic methods, a copper foil is attached to a transparent base and photolithography is employed for etching (PTL 7). This method allows fine processing to produce a mesh with a high open area ratio (high transmittance), while the conductivity is also high. However, photolithographic methods are essentially utilized for processing of small-sized areas and are not easily adaptable for large areas.
Methods using electrodeposited meshes have been proposed in which a mesh-like metal electrodeposition layer having a metal electrolyte solution electrodeposited on an electrodeposition substrate is bonded and transferred to a transparent base (PTL 8). PTL 8 discloses a mesh with a line width of 30 μm. This method produces an easily recognizable thick line-width mesh form. However, it cannot easily produce fine line widths.
Thus, a conductive film with a metal mesh on the surface, produced by prior art technology, has relatively high conductivity but has been problematic in terms of either fine pattern formation or large-area sizes.